Re: [PATCH 4.19.y] PCI: Probe bridge window attributes once at enumeration-time

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Aug 10, 2020 at 05:19:42PM +0300, Dima Stepanov wrote:
> From: Bjorn Helgaas <bhelgaas@google.com>
> 
> commit 51c48b310183ab6ba5419edfc6a8de889cc04521 upstream.
> 
> pci_bridge_check_ranges() determines whether a bridge supports the optional
> I/O and prefetchable memory windows and sets the flag bits in the bridge
> resources.  This *could* be done once during enumeration except that the
> resource allocation code completely clears the flag bits, e.g., in the
> pci_assign_unassigned_bridge_resources() path.
> 
> The problem with pci_bridge_check_ranges() in the resource allocation path
> is that we may allocate resources after devices have been claimed by
> drivers, and pci_bridge_check_ranges() *changes* the window registers to
> determine whether they're writable.  This may break concurrent accesses to
> devices behind the bridge.
> 
> Add a new pci_read_bridge_windows() to determine whether a bridge supports
> the optional windows, call it once during enumeration, remember the
> results, and change pci_bridge_check_ranges() so it doesn't touch the
> bridge windows but sets the flag bits based on those remembered results.
> 
> Link: https://lore.kernel.org/linux-pci/1506151482-113560-1-git-send-email-wangzhou1@hisilicon.com
> Link: https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg02082.html
> Reported-by: Yandong Xu <xuyandong2@huawei.com>
> Tested-by: Yandong Xu <xuyandong2@huawei.com>
> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Sagi Grimberg <sagi@grimberg.me>
> Cc: Ofer Hayut <ofer@lightbitslabs.com>
> Cc: Roy Shterman <roys@lightbitslabs.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Cc: Zhou Wang <wangzhou1@hisilicon.com>
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=208371
> Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> ---
>  drivers/pci/probe.c     | 52 +++++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/pci/setup-bus.c | 45 ++++--------------------------------------
>  include/linux/pci.h     |  3 +++
>  3 files changed, 59 insertions(+), 41 deletions(-)

Why is this now needed in 4.19.y?  What changed to require it and what
prevents the users from just using 5.4.y instead?

A bit of an explaination when backporting patches that are not obvious
"fixes" to much older kernels is always appreciated :)

thanks,

greg k-h